1. Field of the Invention
The present invention relates to a liquid-crystal display system in which a halftone display is presented by applying two voltages to a pixel of a liquid-crystal display panel alternately in successive frames. More particularly, it relates to a liquid-crystal display system which is most suited to present a flickerless halftone display.
2. Related Art
Heretofore, a method of displaying halftone in a liquid-crystal display system has prevented flicker in such a way that the timing for alternately applying two voltages is different for adjacent lines, as stated in the official gazette of Japanese Patent Application Laid-open No. 195628/1987. With this method, however, when displaying a specified display pattern in which the halftone display is presented every second line by way of example, the effect of preventing the flicker using different timings as mentioned above will be canceled and so flicker is again incurred.
The prior-art technique will be described in detail with reference to FIGS. 63.about.65. In these figures, a black box represents "display-OFF", a hatched box a "halftone display", and a blank box "display-ON".
FIG. 63 is a diagram showing the display patterns of respective frames in the case where four illustrated lines are all displayed at a halftone level by the use of the prior art (hereinbelow, the patterns shall be called "halftone patterns"). Timings for bestowing the display-ON and the display-OFF states are made different for the adjacent lines in such a manner that the odd lines are in the display-OFF state in the odd frames and in the display-ON state in the even frames, whereas the even lines are in the display-ON state in the odd frames and the display-OFF state in the even frames. Thus, the halftone display is presented within a certain area (the four lines in FIG. 63).
FIG. 64 illustrates a display example being the condition which is perceived by the eye when respective frames are successively displayed on an actual display screen. Although the halftone display is presented for all the four lines in the example of FIG. 63, the halftone is displayed only every second line in this example of FIG. 64. FIG. 65 is a diagram showing the display patterns of the respective frames in the case of the display depicted in FIG. 64.
A liquid crystal displays the halftone between white (display-ON) and black (display-OFF) when repeatedly endowed with the display-ON and the display-OFF states alternately in successive frames. However, when adjacent lines are simultaneously in halftone display states, the repetition of display-ON and display-OFF of these lines at the same timings gives rise to flicker. As illustrated in FIG. 63, therefore, the timings are made different between the adjacent lines so as to prevent the liquid crystal display from flickering.
However, in such a case where the odd lines are set at the halftone display and the even lines at display-ON as shown in FIG. 64, the liquid crystal display flickers as seen from FIG. 65 illustrative of the display patterns of the respective frames. More specifically, in the odd frames, the odd lines are in the display-OFF state, and the even lines are in the display-ON state, while in the even frames, all the lines are in the display-ON state, so that only the odd lines repeatedly alternate between display-ON and display-OFF. The prior-art technique mentioned above does not take into consideration the flickering which is ascribable to the interference between the display pattern as shown in FIG. 64 and the timings for alternately applying the two voltages.
Incidentally, each of the official gazettes of Japanese Patent Applications Laid-open No. 2722/1991 and No. 20780/1991 discloses a method of driving a liquid-crystal display system wherein tone or grayscale display of different brightnesses in several steps is accomplished by setting a plurality of frames as one cycle and then ON-driving pixels over the number of frames, which corresponds to a grayscale level of display data, within the cycle. In this method, a plurality of adjacent pixels (for example, four pixels or eight pixels) are combined into one group, and the display data for stipulating a tone is designated in group units. Such a method, is intended to reduce the flickering of the displays. This technique, however, adopts a so-called areal modulation system designating a tone in plural-pixel unit and is not directly applicable to a system designating a tone (halftone) in single-pixel unit.